docs: def preview(ident):
      '''Preview an harvesting for a given source'''
      source = get_source(ident)
      cls = backends.get(current_app, source.backend)
      max_items = current_app.config['HARVEST_PREVIEW_MAX_ITEMS']
      backend = cls(source, dryrun=True, max_items=max_items)
      return backend.harvest()